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Full text of "Mathematical And Physical Papers - Iii"

346          ON THE  CHANGE  OF REFRANGIBILITY  OF  LIGHT.
be seen by analysing the light transmitted through the crystals. The following arrangement exhibited at one view the absorption bands and those due to the light which had changed its re-frangibility.
148.    The sun's light was reflected horizontally by a mirror, and  condensed  by passing through a large lens.    It was then transmitted  through  a  vessel with parallel  sides  containing  a moderately strong ammoniacal solution of a salt of copper.    The strength of the solution, and the length of the path of the light within it, were such as to allow of the transmission of a little green besides the blue and violet,   A crystal of nitrate of uranium was then attached to a narrow slit, and placed in the blue beam which had been transmitted through the  solution, the  crystal being turned towards the incident light.    The light coming from the crystal through the slit was then viewed from behind, and analysed by a prism.    A most remarkable  spectrum  was  thus exhibited,  consisting from   end  to  end   of  nothing  but  bands arranged at regular intervals.    The interval between consecutive bands appeared to increase gradually from the red to the violet, just as is the case with bands of interference.    Although this interval  appeared  to  alter  continuously  from   one  end   of the spectrum to the other, the entire system of bands was made up of two distinct systems, different in appearance, and very different in nature.    The less refrangible part of the spectrum, where only for the crystal there would have been nothing but darkness, was filled with narrow bright  bands, due  to  the  light which   had changed  its  refrangibility.    These  bands  were  much   narrower than the dark intervals between them, but they were not mere lines  containing light of definite refrangibility.    The  more refrangible part  of the spectrum was occupied by the system of bands of absorption.    The interval between the most refrangible bright band and the least refrangible dark band of absorption appeared to be a very little greater than one band-interval, so that had there been one band more of either kind the  least refrangible absorption band would have been situated immediately above the most refrangible bright band.    With strong light I think I have seen an additional band of this nature.
149.   Pitchblende.—This mineral proved to be quite insensible, and exhibited nothing remarkable.